As a utility vehicle which is an example of a work vehicle, the Japanese Unexamined Patent Application Publication No. 2010-53952 (see SM, 12, 59-62, a, b, h in FIG. 7) discloses a utility vehicle having a hydrostatic stepless speed changer apparatus configured as described below. Namely, a hydraulic pump and a hydraulic motor are coupled to each other via a high-pressure oil passage and a low-pressure oil passage. As work oil is fed from the hydraulic motor via the high-pressure oil passage to the hydraulic motor, this hydraulic motor is driven; and the work oil from the hydraulic motor is returned via the low-pressure oil passage to the hydraulic pump. The hydraulic pump is configured as a variable displacement type. So that, as a driver operates a speed changer operational tool, the swash plate of the hydraulic pump is operated in stepless manner to effect a speed changing operation.
In the Japanese Unexamined Patent Application Publication No. 2010-53952, the hydraulic pump too is configured as a variable displacement type, and there is provided a hydraulic cylinder for operating the swash plate of the hydraulic motor. A coil spring is mounted in a bottom side oil chamber of the hydraulic cylinder, so that the hydraulic cylinder is urged to its extending side (the high speed side of the swash plate of the hydraulic motor). A pilot pressure branched from the high-pressure oil passage is fed to a rod-side oil chamber of the hydraulic cylinder.
For instance, as indicated by the one-dot chain line L1 in FIG. 5 for instance, under a normal traveling condition, the hydraulic cylinder is extended by the urging force of the coil spring, whereby the swash plate of the hydraulic motor is operated to its minimal angle (high speed side). As is the case with e.g. an upslope, as the traveling load increases, there occurs rise in the pressure of the high-speed oil passage. Hence, the pilot pressure branched from the high-speed oil passage causes the hydraulic cylinder to be contracted against the urging force of the coil spring, whereby the swash plate of the hydraulic motor is operated to the low-speed side, thus alleviating the load to be applied to the engine.